1. Field of the Invention
This invention relates to instruments and methods for the non-invasive quantitative measurement of blood analytes. More specifically, this invention relates to a method for providing general calibration for near-infrared instruments for measurement of blood analytes.
2. Description of Background Art
Information concerning the chemical composition of blood is widely used to assess the health characteristics of both people and animals. For example, analysis of the glucose content of blood provides an indication of the current status of metabolism. Blood analysis, by the detection of above or below normal levels of various substances, also provides a direct indication of the presence of certain types of diseases and dysfunctions.
A current type of blood glucose analytical instrumentation is available for the specific purpose of determining blood glucose levels in people with diabetes. This technology uses a small blood sample from a finger poke which is placed on a chemically treated carrier and is inserted into a portable battery operated instrument. The instrument analyzes the blood sample and provides a blood glucose level reading in a short period of time.
A different class of blood glucose analytical instruments is the near-infrared quantitative analysis instrument which noninvasively measures blood glucose, such as the type described in copending application Ser. No. 07/565,302. The noninvasive blood glucose measurement instrument analyzes near-infrared energy following interactance with venous or arterial blood, or transmission through a blood-containing body part. These instruments give accurate blood glucose level readings and readily lend themselves to at-home testing by diabetics.
A limitation of the near-infrared blood glucose measurement instruments has been that each instrument may be required to be custom calibrated for each individual user. The need for individual custom calibration results from the different combination of water level, fat level and protein level in various individuals which causes variations in energy absorption. Since the amount of glucose in the body is less than one thousandth of these other constituents, variations of these constituents which exist among different people has made a general or universal calibration appear unlikely.
The current approach for custom calibrating near-infrared blood glucose measurement instruments is to use an in-vitro technique that requires removing blood from the subject and having an automatic instrument measure the glucose level of that blood. Such in-vitro measurements are typically made with either the commercially available Biostator or the experimental Kowarski Continuous Monitor. Each of the above instruments requires a catheter to be inserted into the subject and blood withdrawn over a one to two hour period. Although such an approach is feasible, it places a significant new burden on the doctor and the medical facility to have enough time, room and equipment to be able to calibrate instruments in this fashion.
In another technique, a low-cost method and means is used for providing custom calibration for near-infrared instruments for measurement of blood glucose which comprises obtaining a plurality of blood samples from an individual at a predetermined time interval and for a predetermined period of time. Blood glucose measurements for each blood sample are obtained and are entered into the near-infrared instrument. Noninvasive near-infrared optical absorption measurements are concomitantly taken through a body part of the individual at a second predetermined time interval and are recorded in the analysis instrument. Calibration regression analysis is then performed utilizing means for linearly interpolating the blood sample glucose measurements with the near-infrared optical measurements to custom calibrate the near-infrared instrument for the individual. Although representing a significant advancement in custom calibration, this technique does not permit virtually any user to obtain accurate blood glucose level measurements without first having to individually calibrate the instrument. As a result, individual custom calibration can be a significant burden on time and on medical facilities.
Thus, there is a great need for a technique which allows an individual user to obtain fast and accurate blood glucose level measurements without having to first individually calibrate the analysis instrument.